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<title>编程中的一些概念</title><link href='https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext' rel='stylesheet' type='text/css' /><style type='text/css'>html {overflow-x: initial !important;}:root { --bg-color:#ffffff; --text-color:#333333; --select-text-bg-color:#B5D6FC; --select-text-font-color:auto; --monospace:"Lucida Console",Consolas,"Courier",monospace; }
html { font-size: 14px; background-color: var(--bg-color); color: var(--text-color); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; -webkit-font-smoothing: antialiased; }
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iframe { margin: auto; }
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pre.md-meta-block { font-size: 0.8rem; min-height: 0.8rem; white-space: pre-wrap; background: rgb(204, 204, 204); display: block; overflow-x: hidden; }
p > .md-image:only-child:not(.md-img-error) img, p > img:only-child { display: block; margin: auto; }
p > .md-image:only-child { display: inline-block; width: 100%; }
#write .MathJax_Display { margin: 0.8em 0px 0px; }
.md-math-block { width: 100%; }
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code, pre, samp, tt { font-family: var(--monospace); }
kbd { margin: 0px 0.1em; padding: 0.1em 0.6em; font-size: 0.8em; color: rgb(36, 39, 41); background: rgb(255, 255, 255); border: 1px solid rgb(173, 179, 185); border-radius: 3px; box-shadow: rgba(12, 13, 14, 0.2) 0px 1px 0px, rgb(255, 255, 255) 0px 0px 0px 2px inset; white-space: nowrap; vertical-align: middle; }
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code { text-align: left; vertical-align: initial; }
a.md-print-anchor { white-space: pre !important; border-width: initial !important; border-style: none !important; border-color: initial !important; display: inline-block !important; position: absolute !important; width: 1px !important; right: 0px !important; outline: 0px !important; background: 0px 0px !important; text-decoration: initial !important; text-shadow: initial !important; }
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.MathJax_SVG_Display, .md-inline-math .MathJax_SVG_Display { width: auto; margin: inherit; display: inline-block !important; }
.MathJax_SVG .MJX-monospace { font-family: var(--monospace); }
.MathJax_SVG .MJX-sans-serif { font-family: sans-serif; }
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[lang="mermaid"] .node text { font-size: 1rem; }
table tr th { border-bottom: 0px; }
video { max-width: 100%; display: block; margin: 0px auto; }
iframe { max-width: 100%; width: 100%; border: none; }
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mark .md-meta { color: rgb(0, 0, 0); opacity: 0.3 !important; }


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.cm-s-inner .cm-string-2 { color: rgb(255, 85, 0); }
.cm-s-inner .cm-meta { color: rgb(85, 85, 85); }
.cm-s-inner .cm-qualifier { color: rgb(85, 85, 85); }
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.cm-em { font-style: italic; }
.cm-link { text-decoration: underline; }
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.cm-defined { color: rgb(181, 137, 0); }
div.CodeMirror span.CodeMirror-matchingbracket { color: rgb(0, 255, 0); }
div.CodeMirror span.CodeMirror-nonmatchingbracket { color: rgb(255, 34, 34); }
.cm-s-inner .CodeMirror-activeline-background { background: inherit; }
.CodeMirror { position: relative; overflow: hidden; }
.CodeMirror-scroll { height: 100%; outline: 0px; position: relative; box-sizing: content-box; background: inherit; }
.CodeMirror-sizer { position: relative; }
.CodeMirror-gutter-filler, .CodeMirror-hscrollbar, .CodeMirror-scrollbar-filler, .CodeMirror-vscrollbar { position: absolute; z-index: 6; display: none; }
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.CodeMirror-gutters { position: absolute; left: 0px; top: 0px; padding-bottom: 30px; z-index: 3; }
.CodeMirror-gutter { white-space: normal; height: 100%; box-sizing: content-box; padding-bottom: 30px; margin-bottom: -32px; display: inline-block; }
.CodeMirror-gutter-wrapper { position: absolute; z-index: 4; background: 0px 0px !important; border: none !important; }
.CodeMirror-gutter-background { position: absolute; top: 0px; bottom: 0px; z-index: 4; }
.CodeMirror-gutter-elt { position: absolute; cursor: default; z-index: 4; }
.CodeMirror-lines { cursor: text; }
.CodeMirror pre { border-radius: 0px; border-width: 0px; background: 0px 0px; font-family: inherit; font-size: inherit; margin: 0px; white-space: pre; overflow-wrap: normal; color: inherit; z-index: 2; position: relative; overflow: visible; }
.CodeMirror-wrap pre { overflow-wrap: break-word; white-space: pre-wrap; word-break: normal; }
.CodeMirror-code pre { border-right: 30px solid transparent; width: fit-content; }
.CodeMirror-wrap .CodeMirror-code pre { border-right: none; width: auto; }
.CodeMirror-linebackground { position: absolute; left: 0px; right: 0px; top: 0px; bottom: 0px; z-index: 0; }
.CodeMirror-linewidget { position: relative; z-index: 2; overflow: auto; }
.CodeMirror-wrap .CodeMirror-scroll { overflow-x: hidden; }
.CodeMirror-measure { position: absolute; width: 100%; height: 0px; overflow: hidden; visibility: hidden; }
.CodeMirror-measure pre { position: static; }
.CodeMirror div.CodeMirror-cursor { position: absolute; visibility: hidden; border-right: none; width: 0px; }
.CodeMirror div.CodeMirror-cursor { visibility: hidden; }
.CodeMirror-focused div.CodeMirror-cursor { visibility: inherit; }
.cm-searching { background: rgba(255, 255, 0, 0.4); }
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:root {
    --side-bar-bg-color: #fafafa;
    --control-text-color: #777;
}

@include-when-export url(https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext);

html {
    font-size: 16px;
}

body {
    font-family: "Open Sans","Clear Sans","Helvetica Neue",Helvetica,Arial,sans-serif;
    color: rgb(51, 51, 51);
    line-height: 1.6;
}

#write {
    max-width: 860px;
  	margin: 0 auto;
  	padding: 30px;
    padding-bottom: 100px;
}
#write > ul:first-child,
#write > ol:first-child{
    margin-top: 30px;
}

a {
    color: #4183C4;
}
h1,
h2,
h3,
h4,
h5,
h6 {
    position: relative;
    margin-top: 1rem;
    margin-bottom: 1rem;
    font-weight: bold;
    line-height: 1.4;
    cursor: text;
}
h1:hover a.anchor,
h2:hover a.anchor,
h3:hover a.anchor,
h4:hover a.anchor,
h5:hover a.anchor,
h6:hover a.anchor {
    text-decoration: none;
}
h1 tt,
h1 code {
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}
h2 tt,
h2 code {
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}
h3 tt,
h3 code {
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}
h4 tt,
h4 code {
    font-size: inherit;
}
h5 tt,
h5 code {
    font-size: inherit;
}
h6 tt,
h6 code {
    font-size: inherit;
}
h1 {
    padding-bottom: .3em;
    font-size: 2.25em;
    line-height: 1.2;
    border-bottom: 1px solid #eee;
}
h2 {
   padding-bottom: .3em;
    font-size: 1.75em;
    line-height: 1.225;
    border-bottom: 1px solid #eee;
}
h3 {
    font-size: 1.5em;
    line-height: 1.43;
}
h4 {
    font-size: 1.25em;
}
h5 {
    font-size: 1em;
}
h6 {
   font-size: 1em;
    color: #777;
}
p,
blockquote,
ul,
ol,
dl,
table{
    margin: 0.8em 0;
}
li>ol,
li>ul {
    margin: 0 0;
}
hr {
    height: 2px;
    padding: 0;
    margin: 16px 0;
    background-color: #e7e7e7;
    border: 0 none;
    overflow: hidden;
    box-sizing: content-box;
}

li p.first {
    display: inline-block;
}
ul,
ol {
    padding-left: 30px;
}
ul:first-child,
ol:first-child {
    margin-top: 0;
}
ul:last-child,
ol:last-child {
    margin-bottom: 0;
}
blockquote {
    border-left: 4px solid #dfe2e5;
    padding: 0 15px;
    color: #777777;
}
blockquote blockquote {
    padding-right: 0;
}
table {
    padding: 0;
    word-break: initial;
}
table tr {
    border-top: 1px solid #dfe2e5;
    margin: 0;
    padding: 0;
}
table tr:nth-child(2n),
thead {
    background-color: #f8f8f8;
}
table tr th {
    font-weight: bold;
    border: 1px solid #dfe2e5;
    border-bottom: 0;
    margin: 0;
    padding: 6px 13px;
}
table tr td {
    border: 1px solid #dfe2e5;
    margin: 0;
    padding: 6px 13px;
}
table tr th:first-child,
table tr td:first-child {
    margin-top: 0;
}
table tr th:last-child,
table tr td:last-child {
    margin-bottom: 0;
}

.CodeMirror-lines {
    padding-left: 4px;
}

.code-tooltip {
    box-shadow: 0 1px 1px 0 rgba(0,28,36,.3);
    border-top: 1px solid #eef2f2;
}

.md-fences,
code,
tt {
    border: 1px solid #e7eaed;
    background-color: #f8f8f8;
    border-radius: 3px;
    padding: 0;
    padding: 2px 4px 0px 4px;
    font-size: 0.9em;
}

code {
    background-color: #f3f4f4;
    padding: 0 2px 0 2px;
}

.md-fences {
    margin-bottom: 15px;
    margin-top: 15px;
    padding-top: 8px;
    padding-bottom: 6px;
}


.md-task-list-item > input {
  margin-left: -1.3em;
}

@media print {
    html {
        font-size: 13px;
    }
    table,
    pre {
        page-break-inside: avoid;
    }
    pre {
        word-wrap: break-word;
    }
}

.md-fences {
	background-color: #f8f8f8;
}
#write pre.md-meta-block {
	padding: 1rem;
    font-size: 85%;
    line-height: 1.45;
    background-color: #f7f7f7;
    border: 0;
    border-radius: 3px;
    color: #777777;
    margin-top: 0 !important;
}

.mathjax-block>.code-tooltip {
	bottom: .375rem;
}

.md-mathjax-midline {
    background: #fafafa;
}

#write>h3.md-focus:before{
	left: -1.5625rem;
	top: .375rem;
}
#write>h4.md-focus:before{
	left: -1.5625rem;
	top: .285714286rem;
}
#write>h5.md-focus:before{
	left: -1.5625rem;
	top: .285714286rem;
}
#write>h6.md-focus:before{
	left: -1.5625rem;
	top: .285714286rem;
}
.md-image>.md-meta {
    /*border: 1px solid #ddd;*/
    border-radius: 3px;
    padding: 2px 0px 0px 4px;
    font-size: 0.9em;
    color: inherit;
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<div  id='write'  class = 'is-node'><h1><a name="编程中的一些概念" class="md-header-anchor"></a><span>编程中的一些概念</span></h1><h2><a name="线程安全" class="md-header-anchor"></a><span>线程安全</span></h2><p><span>线程安全就是多条线程访问同一段代码时，不会产生不确定的结果，也就是说多次执行结果应该是一样的。依靠线程</span>
<span>同步来保证线程安全，Java中使用同步锁(Synchronized)来实现线程同步。</span></p><h2><a name="线程同步io同步io阻塞" class="md-header-anchor"></a><span>线程同步、IO同步、IO阻塞</span></h2><p><span>线程的异步就是当出现耗时操作后，程序不必停下来等待，实现线程异步的方式一般是多线程，也就是说多线程就是异步，但同时需要注意线程之间的同步。线程的同步就是当多线程访问相同资源时，同一时间仅有一个线程能访问，保证数据在多个线程之间的同步。</span></p><p><span>IO的同步与异步指的是IO操作的消息通知方式。</span><br/><span>同步IO：线程在发起IO请求后会等待IO的返回结果</span><br/><span>异步IO：线程在发起IO请求后不等待结果，操作系统完成IO操作后，主动通知线程</span><br/><span>阻塞IO：发起IO请求后，线程不能进行其他操作，死等IO操作完成</span><br/><span>非阻塞IO：线程发起IO请求后，不等待，继续其他操作</span></p><p><span>同步与阻塞不是相同的概念，同步的IO可以是非阻塞的，比如线程发起IO请求后，系统虽然还没有完成IO操作，但是立即返回结果（比如返回ERROR），然后系统继续执行IO操作，线程继续后续操作不再等待IO，那线程怎么知道IO什么时候完成呢，就是采用轮询，每隔一段时间再去查询一下上次的那个IO操作是否已经完成，知道IO返回OK为止。</span></p><p><span>Java的NIO就是同步非阻塞的IO，python可以用asyncio实现异步非阻塞IO。</span></p><h2><a name="回调" class="md-header-anchor"></a><span>回调</span></h2><p><span>网上对于回调的说法很乱，而且举的例子常常给我一种错觉，以为回调是A在调用B的同时，B反过来调用A，比如网上的例子常常是这样的：</span><br/><span>A对象调用B对象的方法C，同时将自己作为参数传给方法C，然后方法C再调用A的方法B。</span><br/><span>我认为这种理解并不准确。</span><br/><span>回调的文字表述是——“don&#39;t call me, I&#39;ll call you back”，更生活化的理解是“不要问我这件事做完了没，我做完了会告诉你的”。</span></p><p><span>我觉得回调的流程应该是： A告诉B在某个时刻调用A。到了这个约定的时刻，B调用A。</span><br/><span>这里A和B调用的不一定是对方的方法，也可能是对方全局或局部变量的方法。</span><br/><span>举个例子，比如Android中我们常常在Activity中为button设置监听器。  </span></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="java"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">button</span>.<span class="cm-variable">setOnClickListener</span>(<span class="cm-keyword">new</span> <span class="cm-variable">OnClickListener</span>(){</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">　　<span class="cm-meta">@Override</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">　　<span class="cm-keyword">public</span> <span class="cm-variable-3">void</span> <span class="cm-variable">OnClick</span>(<span class="cm-variable">Button</span> <span class="cm-variable">b</span>) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">　　　　<span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"clicked"</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">　　}</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">});</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 138px;"></div><div class="CodeMirror-gutters" style="display: none; height: 138px;"></div></div></div></pre><p><span>我现在假设这个Activity的名字就activity，来看这个流程：</span><br/><span>activity调用button的setOnClickListener对象，告诉button我有一个局部的匿名内部类对象（也就是继承自OnClickListener的这个对象），你在被点击时记得调用它的OnClick方法。于是当button对象被点击时，这个onClick方法被调用了。</span><br/><span>这样就可以理解为什么Android中onXXX方法都被叫做回调方法。</span><br/><span>还有一个问题是类似与Activity生命周期的方法，Activity好像并没有调用哪个对象的方法，也没把自己传给哪个对象，好让它在适当的时机来调用自己的生命周期方法。</span><br/><span>对于这个我想Activity肯定通过什么方法注册了自己，也许就是AndroidManifest文件，来告诉别的类（ActivityManagerService？）——(｡･∀･)ﾉﾞ嗨，这里有一个Activity哦，记得在适当的时候调用我的生命周期方法。</span></p><h2><a name="event-hook" class="md-header-anchor"></a><span>event hook</span></h2><p><a href='https://stackoverflow.com/questions/11087543/what-is-the-difference-between-hook-and-callback'><span>What is the difference between hook and callback?</span></a></p><p><span>来自stack overflow：</span><br/><span>callback：回调，一个函数注册到一个API上，在适当的时机这个函数会被回调。</span><br/><span>event hook：事件钩子，或者直接叫做hook，可以理解为更通用的回调，它可以修改API的调用，比如修改传递给API的参数。</span></p><p><span>callback是一种特殊的hook，这两者的区别比较模糊，在有些语言中甚至hook就是callback。</span></p><h2><a name="方法与函数" class="md-header-anchor"></a><span>方法与函数</span></h2><p><span>我以前一直以为方法和函数是同一种东西的不同叫法，就和句柄、引用，可是事实并不是如此。</span><br/><span>不同语言对于函数与方法的区别是不同的，比如java只有方法，而c++只有函数，类中的函数被称为成员函数。</span><br/><span>Python中方法与函数的区别是，绑定了非type类型的对象的就是方法，否则就是函数。</span></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="python"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">class</span> <span class="cm-def">TestClass</span>():</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">def</span> <span class="cm-def">func</span>(<span class="cm-variable-2">self</span>): &nbsp; &nbsp;<span class="cm-comment"># 定义在类里面的，这也是一个函数</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp; &nbsp; &nbsp;<span class="cm-string">'''func'''</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp; &nbsp; &nbsp;<span class="cm-keyword">return</span> <span class="cm-keyword">lambda</span> <span class="cm-variable">x</span>:<span class="cm-variable">x</span><span class="cm-operator">+</span><span class="cm-number">2</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">a</span>=<span class="cm-variable">TestClass</span>() <span class="cm-comment"># 只有当函数与非type类型的对象绑定时，才会变成方法</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-builtin">print</span>(<span class="cm-builtin">type</span>(<span class="cm-variable">TestClass</span>.<span class="cm-property">func</span>)) <span class="cm-comment"># 这里输出为 &lt;class 'function'&gt;</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-builtin">print</span>(<span class="cm-builtin">type</span>(<span class="cm-variable">a</span>.<span class="cm-property">func</span>)) <span class="cm-comment"># 这里输出 &lt;class 'method'&gt;</span></span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 161px;"></div><div class="CodeMirror-gutters" style="display: none; height: 161px;"></div></div></div></pre><h2><a name="内存泄漏和内存溢出" class="md-header-anchor"></a><span>内存泄漏和内存溢出</span></h2><p><span>内存泄漏，即部分对象虽然已经不再使用，但是因为有root持有引用，所以并没有被销毁，所占用的内存一直没有被释放。一次两次发生影响不大。如果频繁发生，那么可用内存会渐渐不足，最终在某一次请求内存时发现内存不足而发生oom（即Java的OutOfMemoryError）。这里要明确一个概念，只有强引用会发生内存泄漏，而weak等引用因为其特殊机制，所以影响不大。</span><br/><span>详见</span><a href='https://zhuanlan.zhihu.com/p/28169456'><span>Android内存泄漏学习笔记</span></a></p><p><span>两者的区别：</span><br/><span>内存溢出 out of memory，是指程序在申请内存时，没有足够的内存空间供其使用，出现out of memory；比如申请了一个integer,但给它存了long才能存下的数，那就是内存溢出。</span><br/><span>内存泄露 memory leak，是指程序在申请内存后，无法释放已申请的内存空间，一次内存泄露危害可以忽略，但内存泄露堆积后果很严重，无论多少内存,迟早会被占光。</span></p><h2><a name="行尾符" class="md-header-anchor"></a><span>行尾符</span></h2><p><span>在打字机时代换行符（LF \r 换到下一行）和回车符（CR \n 光标移到行首 ）是两个不同的符号，但是到了计算机时代，回车这一个字符即可做到原先换行回车两个字符才能实现的效果。</span><br/><span>Windows 沿用了打字机时代的做法，用 CRLF（\n\r）在行尾表示换行。 Linux 使用 LF（\n）在行尾表示换行。</span><br/><span>但是由于大量的编程语言使用 \n 作为换行，所以其实在 Windows 中编程的时候，它会自动对 CRLF 和 LF 做转换。</span></p><h3><a name="utf-8-与-bom" class="md-header-anchor"></a><span>UTF-8 与 BOM</span></h3><p><span>Linux 的 UTF-8 不带 BOM，而 windows 的带。Windows 特有，所以还是使用不带 BOM 的编码，这样更通用一些。</span></p><p><span>这个 BOM 就是文件开头的一个特殊字符，用来表明当前使用的是哪一种 Unicode 编码（UTF-8，UTF-16BE，UTF-16LE）。Windows 与 Linux 在这方面的区别有很多原因，其中一个原因是微软对于兼容性的高要求，微软希望系统能兼容更多的字符编码（想想 Linux 经常出现中文乱码）。</span></p><h2><a name="二进制" class="md-header-anchor"></a><span>二进制</span></h2><p><span>说来惭愧，大学毕业连二进制都没搞懂。</span><br/><span>我这里会用一些实际问题来描述二进制的概念。</span></p><p><span>二进制第一个是符号位，正数为0，负数为1。</span><br/><span>正数补全是往左边添0，负数则是添1。</span><br/><span>0001 → 0000 0001。</span><br/><span>1001 → 1111 1001。</span></p><h3><a name="二进制的表示范围" class="md-header-anchor"></a><span>二进制的表示范围</span></h3><p><span>八位二进制的话能表示 7~-8，我从大学开始就不清楚，为什么负数能多表示一个，对于正数而言，最大值是 0111；我当时想当然地以为负数的最小值是 1111（-7），确实如此，可是计算机中存储的是二进制的补码，不是原码。</span></p><p><span>正数的补码就是本身。</span><br/><span>负数的补码是在原码的基础上对数值位取反后+1，符号位不变，所以 -7 在计算机中的表示是 1001，而 -1 的表示是 1111。</span><br/><span>负数能多一个的原因是因为0只有一种表示形式，就是+0（0000），对于-0（1000）而言，对它取补码也变成了 1 0000，这是个四位二进制数，所以最左边的1被舍弃变成了 0000。</span></p><p><span>一个四位二进制，最多16种变化，因为0使用了正数的符号位，所以正数少一个变化。</span><br/><span>多出来的-8，用哪个表示呢？用-1（1111）不断地-1，减到最后能发现最小的是1000，也就是-8，也就是-0的原码。或者另一种思路是用-8的原码（1 1000），求补码后再舍弃最左边的一位，也可以得到（1000）。</span></p><h3><a name="int溢出" class="md-header-anchor"></a><span>int溢出</span></h3><p><span>最大的int值+1后会变成最小的int值，同理，最小的int值-1会变成最大的int值。</span><br/><span>简单的说就是数值位能表示的数为【最大/最小】时，再对其【加上/减去】数值，使得符号位发生了【进位/借位】。</span></p><p><span>0111（7） + 0001 = 1000（-8）</span><br/><span>1000（-8） - 0001 = 0111（7）</span></p><h2><a name="缓冲buffer）与缓存cache）" class="md-header-anchor"></a><span>缓冲（buffer）与缓存（cache）</span></h2><p><a href='https://www.zhihu.com/question/26190832'><span>Cache和Buffer都是缓存，主要区别是什么？</span></a></p><p><span>buffer（缓冲）是为了减少短时间内发起的IO次数，如果有100的字节，分100次写入到文件，就要浪费大量的IO资源，这时可以用一个缓冲区来存储这些资源，每存到10个后，写入一次，这样总共就写了10次。</span><br/><span>cache（缓存）位于速度相差较大的两种硬件之间，用于协调两者数据传输速度差异的结构。就是把高速设备需要频繁访问的数据放在cache中，数据先在cache中查找，没有的话，再去低速设备中查找。就是为了加快读取速度。</span></p><p><span>buffer是从内存中划分出来的空间，cache也可以是从内存中划分出来的，但是cache有专门的硬件，比如CPU都会带三级缓存，硬盘也有板载缓存，但是我没听说有什么专门的硬件是来支持buffer的。</span></p><h2><a name="表达式和语句" class="md-header-anchor"></a><span>表达式和语句</span></h2><p><span>编程中常常听到这两个概念。</span><br/><span>表达式（expression）是值、变量和运算符的组合，表达式一定会有返回值，表达式可以作为赋值语句右边的语句。</span><br/><span>语句包含表达式，还有其他诸如if语句，while语句。</span></p><h2><a name="命名空间和作用域" class="md-header-anchor"></a><span>命名空间和作用域</span></h2><p><span>命名空间就是保存有变量名字和变量值的一个容器，在 python 中是一个字典。python 的命名空间本身也属于更外层的一个命名空间，也就是说一个字典是另一个字典的元素，这样实现了命令空间的嵌套。</span></p><p><span>作用域是一个代码块，在这个代码块中，可以直接访问部分命名空间中的变量，比如 python 中可按照 L-E-G-B 的顺序访问命名空间中的变量。</span><br/><span>所谓直接访问，就是直接使用变量名字，而不是【对象.变量】这种形式。</span></p></div>
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